Process and catalyst for producing syndiotactic polymers

ABSTRACT

Syndiospecific catalysts and processes for the syndiotactic propagation of a polymer chain derived from an ethylenically unsaturated monomer which contains 3 or more carbon atoms or is a substituted vinyl compound. The catalysts comprise an unbalanced metallocene cation characterized by a cationic metallocene ligand having sterically dissimilar ring structures joined to a positively charged coordinating transition metal atom, and a stable noncoordinating counter anion for the metallocene cation. The ring structures are substituted cyclopentadienyl rings which are sterically different from one another at sufficiently low kinetic energy states induced by the substituents on the cyclopentadienyl groups to impart a stereorigid relationship relative to said coordinating metal atom to prevent rotation of the rings about their coordination axes at the polymerization temperature. The catalyst with a C3+ alpha olefin or ther ethylenically unsaturated compound in a polymerization reaction zone and maintained in contact with the catalyst in the reaction zone under low termperature polymerization conditions to produce a syndiotactic polymer.

This application is a division of application Ser. No. 873,445, filed Apr. 24, 1992, now U.S. Pat. No. 5,223,468, which is a continuation of application Ser. No. 418,846, filed Oct. 10, 1989, now abandoned, which, in turn, is a continuation-in-part of application Ser. No. 220,007, filed Jul. 15, 1988, and now U.S. Pat. No. 4,892,851.

TECHNICAL FIELD

This invention relates to catalysts and processes for the production of syndiotactic polymers from ethylenically unsaturated compounds and more particularly to the production of a syndiotactic polyolefin by polymerization of propylene or higher alpha olefin over a stereorigid cationic metallocene catalyst having dissimilar cyclopentadienyl rings.

BACKGROUND OF THE INVENTION

Syndiotacticity is one of a number of stereospecific structural relationships which may be involved in the formation of the stereoregular polymers which may be derived from various monomers. Stereospecific propagation may be applied in the polymerization of ethylenically unsaturated monomers such as C3+ alpha-olefins, 1-dienes such as 1,3-butadiene and additional or substituted vinyl compounds such as vinyl aromatics, e.g., styrene or vinyl chloride, vinyl ethers such as alkyl vinyl ethers, e.g., isobutyl vinyl ether, or even aryl vinyl ethers. Stereospecific polymer propagation is probably of most significance in the production of polypropylene of isotactic or syndiotactic structure.

Syndiotactic polymers have a unique stereochemical structure in which monomeric units having enantiomorphic configuration of the asymmetrical carbon atoms follow each other alternately and regularly in the main polymer chain. Syndiotactic polypropylene was first disclosed by Natta et al. in U.S. Pat. No. 3,258,455. As disclosed in this patent, syndiotactic polypropylene can be produced by using a catalyst prepared from titanium trichloride and diethyl aluminum monochloride. A later patent to Natta et al., U.S. Pat. No. 3,305,538, discloses the use of vanadium triacetylacetonate or halogenated vanadium compounds in combination with organic aluminum compounds for producing syndiotactic polypropylene. U.S. Pat. No. 3,364,190 to Emrick discloses the use of a catalyst system composed of finely divided titanium or vanadium trichloride, aluminum chloride, a trialkyl aluminum and a phosphorus-containing Lewis base in the production of syndiotactic polypropylene.

As disclosed in these patent references and as known in the art, the structure and properties of syndiotactic polypropylene differ significantly from those of isotactic polypropylene. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: ##STR1##

Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.

In contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Syndiotactic polypropylene shown in zig-zag representation as follows: ##STR2##

Corresponding representations for syndiotactic polyvinylchloride and polystyrene respectively are: ##STR3## Using the Fischer projection formula, the structure of a syndiotactic polymer or polymer block for polypropylene is designated as: ##STR4## In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each "r" represents a "racemic" dyad, i.e., successive methyl groups on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer that is soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.

While it is possible for a catalyst to produce all three types of polymers, it is desirable for a catalyst to produce predominantly isotactic or syndiotactic polymer with very little atactic polymer. Catalysts that produce isotactic polyolefins are disclosed in copending U.S. patent application Ser. Nos. 034,472 filed Apr. 3, 1987 now abandoned; 096,075 filed Sep. 11, 1987 and now U.S. Pat. No. 4,794,096; and 095,755 filed on Sep. 11, 1987 and now U.S. Pat. No. 4,975,403. These applications disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of a highly isotactic polypropylene.

Catalysts that produce syndiotactic polypropylene or other syndiotactic polyolefins are disclosed in the aforementioned U.S. Pat. No. 4,892,851. These catalysts are bridged stereorigid metallocene catalysts. The catalysts have a structural bridge extending between dissimilar cyclopentadienyl groups and may be characterized by the formula:

    R"(CpRn)(CpR'm)MeQk                                        (1)

In formula (1),

Cp represents a cyclopentadienyl or substituted cyclopentadienyl ring; and R and R' represent hydrocarbyl radicals having 1-20 carbon atoms.

R" is a structural bridge between the rings imparting stereorigidity to the catalyst; Me represents a transition metal and Q a hydrocarbyl radical or halogen.

R'm is selected so that (CpR'm) is a sterically different substituted cyclopentadienyl ring than (CpRn); n varies from 0 to 4 (0 designating bo hydrocarbyl groups, i.e. an unsubstituted cyclopentadienyl ring) and m varies from 1-4, and K is from 0-3.

The sterically different cyclopentadienyl rings produces a predominantly syndiotactic polymer rather than an isotactic polymer.

Metallocene catalysts of yet another type are cationic catalysts as disclosed in European Patent Applications 277,003 to Turner et al. and 277,004 to Turner. As disclosed in these applications, a bis(cyclopentadienyl) zirconium, titanium or hafnium compound is reacted with a second compound comprising a cation capable of donating a proton or an ion exchange compound comprising a cation which will irreversible react with a ligand on the first compound, and a bulky, stable anion. The catalysts described in the European Patent Applications 277,003 and 277,004 are disclosed as especially useful in the polymerization of ethylene and more generally in the polymerization of alpha olefins, diolefins and/or an acetylenically unsaturated compounds containing from 2-18 carbon atoms. Principally disclosed in the European applications is the polymerization of ethylene or the copolymerization of ethylene with propylene or 1-butene or with propylene and 1-butene or 1,4 hexadiene. Stereospecificity, or lack thereof, of the polymers as disclosed in the Turner and Turner et al. applications is not generally discussed, although in Application 277,004 examples are given of producing atactic polypropylene and in one instance (Example 39) isotactic polypropylene.

SUMMARY OF THE INVENTION

In accordance with the present invention, there are provided syndiospecific catalysts and processes for the syndiotactic propagation of a polymer chain derived from an ethylenically unsaturated monomer which contains 3 or more carbon atoms or is a substituted vinyl compound. Catalysts in accordance with the present invention comprise an unbalanced metallocene cation and a stable noncoordinating counter anion for the metallocene cation. The metallocene cation is characterized by a cationic metallocene ligand having sterically dissimilar ring structures joined to a positively charged coordinating transition metal atom. The ring structures are substituted cyclopentadienyl rings and are sterically different from one another. Both of said cyclopentadienyl groups are at sufficiently low kinetic energy states induced by the substituents on the cyclopentadienyl groups to impart a stereorigid relationship relative to said coordinating metal atom to prevent rotation of the rings about their coordination axes at the polymerization temperature.

Syndiotactic polypropylene or other polymers resulting from the polymerization of C3+ alpha olefins or vinyl compounds may be produced in accordance with the method of the present invention. Syndiospecific propagation of the polymer chain is carried out in the presence of a stereorigid cationic metallocene catalyst which dissimilar substituted cyclopentadienyl rings which are in a stereorigid relationship relative to the coordinating metal atom of the metallocene complex. The catalyst is contacted with a C3+ alpha olefin or other ethylenically unsaturated compound in a polymerization reaction zone and maintained in contact with the catalyst in the reaction zone under low temperature polymerization conditions to produce a syndiotactic polymer. The preferred application of the invention is in the production of syndiotactic polypropylene. Specific catalysts employed in which the aforementioned stereorigid relationship between the dissimilar cyclopentadienyl rings is established in accordance with the present invention may be characterized by the following formula:

    [(CpTx)(CpT'y)MeQk]+P.sub.a.sup.-                          ( 2)

Wherein:

Cp is a substituted cyclopentadienyl ring; each T is the same or different and is a hydrocarbyl radical having from 1-20 carbon atoms;

each T' is the same or different and is a hydrocarbyl radical having from 1-20 carbon atoms and selected such that CpTx and CpT'y are sterically different rings and are both at sufficiently low kinetic energy states induced by the substituent groups T and T' so as to prevent rotation of said rings about their coordination axes at the polymerization temperature.

Q is a hydrocarbyl group having 1-20 carbon atoms or is a halogen;

Me is a Group 4, 5, or 6 metal from the Periodic Table of Elements;

x is from 1 to 5; y is from 1 to 5; k is from 0 to 2;

P_(a) is a stable noncoordinating counter anion.

DETAILED DESCRIPTION

The present invention involves certain stereorigid cationic metallocenes and their use as catalysts in syndiotactic polymer propogation. The term metallocene as used herein and in accordance with normal art usage denotes an organometallic coordination compound in which two cyclo-C5 ligands (cyclopentadienyl or substituted cyclopentadienyl rings) are bonded to a central or "sandwiched" metal atom which may be provided by a transition metal or metal halide, alkyl, alkoxy, or alkyl or alkoxy halide or the like. Such structures are sometimes referred to as "molecular sandwiches" since the cyclo-C5 ligands are oriented above or below the plane of the central coordinated metal atom. By the term "cationic metallocene" is meant a metallocene in which the central coordinated metal atom carries a positive charge, that is, the metallocene complex is a cation associated with a stable anion. The cationic metallocenes involved in the present invention are stereorigid. Stereorigidity is imposed by means of substituted cyclopentadienyl rings in which the substituent groups are such as to provide a state of low kinetic energy of the substituted cyclopentadienyl rings. This low kinetic energy state prevents rotation of the substituted cyclopentadienyl rings about their coordination axes with the metal atom at the temperature of the polymerization reaction.

As noted previously, Parent U.S. Pat. No. 4,892,851 discloses the preparation of syndiotactic polypropylene, or other polyolefins, through the use of stereorigid metallocene catalysts. The present invention employs stereorigid metallocene catalysts, which have dissimilar cyclopentadienyl groups of the type disclosed in Parent U.S. Pat. No. 4,892,851 but in which stereorigidity is imparted without a bridge structure and in which the metallocene ligand is ionized to provide a stable cationic catalyst. The cationic metallocene catalysts involved in the present invention may be prepared following procedures of the type disclosed in the aforementioned European Applications 277,003 and 277,004, but they preferably are prepared by a process employing a triphenylcarbenium boronate as discussed in greater detail below. Where procedures of the type disclosed in the European applications are used in the preparation of cationic metallocene catalysts involved in the present invention, certain important distinctions must be observed as evidenced by the fact that neither of the European applications disclose the preparation of syndiotactic polymers. Thus, in the metallocene catalysts disclosed in the European applications, the cyclopentadienyl groups may be the same or different, and while they can be bridged thus imparting stereorigidity, they need not be and, in fact, are usually unbridged. Moreover, to the extent that the metallocene catalysts disclosed in the European applications are bridged to impart stereorigidity, they are also symmetrical. In contrast to the teachings of the Turner European applications, the cationic metallocene catalysts employed in the present invention must not only be stereorigid, the cyclopentadienyl groups must be dissimilar. Stereorigid cationic metallocene catalysts employed in the present invention may be characterized by the following formula:

    [(CpTx)(CpT'y)MeQk]+P.sub.a.sup.-                          (2)

wherein: Cp, T, T', Me, Q, P_(a), k, x and y are as described previously.

In the catalysts of this formula, stereorigidity is imparted by substituent groups of the cyclopentadienyl rings which are of a nature to impart a relatively low kinetic energy to the cyclopentadienyl rings by virtue of their bulk increasing inertia of the rings, or by way of intramolecular rotational force which counteracts the rotational energy of the rings. In employing the catalysts of the present invention, the polymerization reaction is carried out under low temperature conditions as described in greater detail below. At the polymerization conditions, the two cyclopentadienyl groups are in a sterically hindered relationship sufficient to prevent rotation of the cyclopentadienyl rings, thus imparting stereorigidity to the catalyst.

The counter anion indicated by P_(a) is a compatible noncoordinating anion which may be of the type described in the aforementioned Turner European applications. The anion P_(a) either does not coordinate with the metallocene cation or is only weakly coordinated to the cation thereby remaining sufficiently labile to be displaced by a neutral Lewis base. As described in the Turner applications, the term "compatible noncoordinating anion" identifies an anion which when functioning as a stabilizing anion in the metallocene catalyst system does not transfer an anionic substituent or fragment thereof to the cation to form a neutral metallocene and boron byproduct or other neutral metal or metalloid byproduct, as the case may be. Suitable noncoordinating anions include: [W(PhF5)]-, [Mo(PhF5)-] (wherein PhF5 is pentafluoryl phenol) [ClO4]-, [PF6]-,[SbR6]-, [AlR4]- (wherein each R is independently, C1, a C1-C5 alkyl group, preferably a methyl group, an aryl group, e.g., a phenyl or substituted phenyl group, or a fluorinated aryl group. For a further description of compatible noncoordinating anions and their associated cations which may be employed in the present invention, reference is made to European applications 277,003 and 277,004, the entire disclosures of which are incorporated herein by reference. In considering these disclosures, it must be recalled, however, that unlike the cationic metallocene catalyst of the Turner European applications, the cationic metallocene catalysts employed in the present invention must be stereorigid with dissimilar Cp rings. The size of the counter ion will depend on the bulk of the substituent groups on the cyclopentadienyl rings. The basic requirement for production of the syndiotactic polymers is that the cyclopentadienyl rings be dissimilar and substituted to provide stereorigidity. Monomer insertion and isomerization is controlled by the relationship of the anionic counterion to the stereorigid metallocene ligand.

Stereorigidity is imparted, as noted above, by the reduced kinetic energy state of the rings which may be supplemented by means of direct steric hindrance imparted by the cyclopentadienyl substituent groups. Relatively more bulky substituent groups are required than for bridged structures and steric relationships are developed not only between the cyclopentadienyl substituents but also between the substituents and the noncoordinating anion. Here, the size of the anionic counterion may be slightly smaller than in bridged structures where steric hindrance is not significant, or at least is not as significant, as in the nonbridged structures of the present invention. In addition to size, the other important characteristics of the anionic counterions are stability and bonding. The anion must be sufficiently stable so that it cannot be rendered neutral by virtue of the metallocene cation extracting an anionic substituent or fragment. The bond strength with the cation is such that it must be noncoordinating or only weakly coordinating with the metallocene cation so that it makes way for the inserting monomer in the chain growing reaction.

The metallocene catalysts disclosed in the Turner European applications suffer from certain disadvantages in that Lewis bases may be produced by protonation of the metallocene ligand which function as poisons for the metallocene catalyst. A preferred procedure for producing cationic metallocene catalyst of the type employed in the present invention involves the reaction of an anionic compound in a noncoordinating solvent with a dimethyl metallocene which is unbalanced and stereorigid at polymerization conditions as described because of the substituent groups on the cyclopentadienyl rings. By way of example, triphenylcarbenium tetrakis (pentafluorophenyl) boronate may be reacted with a neutral metallocene precurser in a solvent such as toluene. Such catalysts and their preparation are disclosed in U.S. patent application Ser. No. 419,046 now abandoned by John A. Ewen and Michael J. Elder for "Preparation of Metallocene Catalysts for Polymerization of Olefins" filed on even date herewith, the entire disclosure of which in incorporated by reference.

A preferred application of the invention is in the syndiotactic polymerization of C3+ alpha olefins, specifically propylene, but the invention may be employed in the preparation of other polymers from ethylenically unsaturated monomers where syndiotacticity is a desired structure. By the term ethylenically unsaturated monomer as used herein is meant a hydrocarbon or substituted hydrocarbon compound characterized by a terminal vinyl group (CH2═CH--). Such compounds as may be employed in the present invention have at least three carbon atoms or are a substituted vinyl compound, specifically vinyl chloride. They may be characterized in terms of the following formula:

    CH2═CH--R                                              (3)

wherein: R is a hydrocarbyl group or nonhydrocarbyl substituent. For example, syndiospecific propagation of a polymer chain from 1-butene may be carried out in accordance with the invention. Specific polymers in which syndiotacticity is sometimes desirable and to which the invention is applicable include polyvinyl chloride and polystyrene. The polymerization of a 1-diene such as 1,3-butadiene may also be carried out in accordance with the present invention to achieve a syndiotactic polymer configuration. Syndiotactic polypropylene is probably of the greatest practical significance and the invention will be described in detail with reference to the production of syndiotactic polypropylene. However, other compounds in which the syndiotactic configuration is desirable are also of interest.

Polymerization procedures as disclosed in the aforementioned parent U.S. Pat. No. 4,892,851 may be employed in carrying out the present invention. Co-catalysts, usually organo-aluminum compounds such as trialkylaluminum, trialkyloxyaluminum, dialkylaluminum halides or alkylalumium dihalides may be employed in the present invention. Especially suitable alkylaluminums are trimethylaluminum and triethylaluminum with the latter, commonly referred to as TEAL, being most preferred. However, aluminoxane which may be used as a co-catalyst in the parent U.S. Pat. No. 4,892,851 need not be, and preferably is not, used in carrying out the present invention. While applicants' invention is not to be restricted by theory, it is believed that neutral metallocenes of the type disclosed in the parent application form cationic complexes by reaction with the aluminoxane in the manner as disclosed by Zambelli, A. et al., "Isotactic Polymerization of Propene: Homogenous Catalysts Based on Group 4 Metallocenes Without Methylaluminoxane", Macro-Molecules 1989, 22, pages 2186-2189. It is believed that the anionic species derived from the aluminoxane compound may function to stabilize the cationic metallocene to permit monomer insertion and chain migration and isomerization during the growth of the polymer chain resulting in syndiotacity. The stereorigid cationic metallocene catalysts employed in the present invention accomplish isomerization during monomer insertion and chain migration.

The procedures and reaction conditions disclosed in the aforementioned U.S. Pat. No. 4,892,851 may be employed in the present invention with the exception, as noted above, that aluminoxanes need not be used and preferably are not used. The prior art discloses the use of aluminoxanes as co-catalysts with metallocene catalysts in amounts well in excess of a stoichiometric equivalent amount providing mole ratios of aluminum to the coordinating metal (Me) of about 100-1000. Aluminoxanes usually are not employed in the present invention and if they are used they are in amounts well below the aforementioned range and preferably providing an A1/Me mole rate of no more than 10 and, more preferably, no more than 1.

The catalysts used in the present invention are syndio-specific and produce a polymer with a high syndiotactic index. As disclosed in U.S. Pat. No. 4,892,851, syndiotactic polymers generally have lower heats of crystallization than the corresponding isotactic polymers. In addition, for the same number of imperfections in the polymer chain, syndiotactic polymers have a higher melting point than isotactic polymers.

The metallocene catalysts used in the present invention may be characterized by formula (2) as described above. Me is a Group 4, 5, or 6 metal from the Periodic Table of Elements but preferably is a Group 4 or 5 metal and more preferably a Group 4 metal, specifically titanium, zirconium or hafnium. Vanadium is the most suitable of the Group 5 metals. Each Q is a hydrocarbyl radical having 1-20 carbon atoms or is a halogen. As a practical matter, Q will usually be a methyl or ethyl group or a halide, preferably chlorine. In order to be syndiospecific, the Cp rings in the metallocene catalysts must be substituted in a substantially different manner so that there is a steric difference between the two Cp rings, and therefore, T'y is selected such that (CpT'y) is a substantially different substituted ring than (CpTx). In order to produce a syndiotactic polymer, the characteristics of the groups substituted directly on the cyclopentadienyl rings appear important. Thus, by "steric difference" or "sterically different" as used herein, it is intended to denote a difference between the steric characteristics of the Cp rings that controls the approach of each successive monomer unit that is added to the polymer chain. The steric difference between the Cp rings acts to block the approaching monomer from a random approach and controls the approach such that the monomer is added to the polymer chain in the syndiotactic configuration.

Without intending to limit the scope of the present invention as indicated by the claims, it is believed that in the polymerization reaction, both the catalyst and the approaching monomer units isomerize with each monomer addition to the polymer chain as the chain migrates between catalyst sites. This isomerization of the monomer which is controlled by the steric blockage of the differently substituted Cp rings results in the alternating configuration characteristic of syndiotactic polymers and is in contrast to the chain-end control of the catalysts disclosed by Natta et al. The different reaction mechanism also results in a different structure for the polymer.

In the preferred catalysts for use in the present invention, Me is titanium, zirconium or hafnium; Q is a hydrocarbyl group, preferably methyl, or halogen, preferably chlorine; and k is preferably 1, but may vary with the valence of the metal atom. Exemplary hydrocarbyl radicals in addition to methyl include ethyl, propyl, isopropyl, butyl, isobutyl, amyl, isoamyl, hexyl, heptyl, octyl, nonyl, decyl, cetyl, phenyl, and the like. Other hydrocarbyl radicals useful in the present catalysts include other alkyl, aryl, alkenyl, alkylaryl or arylalkyl radicals. Further, Tx and T'y may comprise hydrocarbyl radicals attached to a single carbon atom in the Cp ring as well as radicals that are bonded to two carbon atoms in the ring. The catalysts used in the present invention may be derived from neutral metallocene moieties which are then converted to the cationic state, following procedures such as disclosed in the aforementioned European applications 277,003 and 277,004 or more preferably by reaction with triphenylcarbenium boronates as described in the aforementioned copending application Ser. No. 419,046 by Ewen and Elder.

As indicated by formula (2), stereorigid metallocene catalysts involved in the present invention are characterized by metallocenes incorporating substituted cyclopentadienyl rings in which the substituent groups are relatively bulky so as to impart a relatively low kinetic energy state to the cyclopentadienyl rings. The nonbridged cationic metallocenes catalyst are further stabilized by carrying out the polymerization reaction under relatively low temperature conditions which further act to retard rotation of the substituted cyclopentadienyl rings about their coordination axes. The requisite low kinetic energy state may be imparted to the cyclopentadienyl rings by substituent groups which are in themselves relatively bulky and reduce the kinetic energy of the cyclopentadienyl rings by virtue of their atomic mass. This effect can be characterized as strictly a physical phenomenon in the sense that by adding the relatively bulky substituent groups to the cyclopentadienyl rings, the inertia of each cyclopentadienyl ring is enhanced so that more energy is needed to change its rotational position relative to the coordination axis to the transition metal. Examples of metallocenes depicted by formula (2) include aryl substituted ligand structures specifically cyclopentadienyl rings with four or five aryl substituents such as benzyl and phenyl groups. Specific examples of metallocene ligands coordinated with a transition metal as described previously include (pentabenzylcyclopentadienyl) (penta phenyl cyclopentadienyl), (tetra phenyl cyclopentadienyl) (pentabenzylcyclopentadienyl). As indicated previously, the preferred transition metals in formulating the metallocene catalysts are zirconium, hafnium and titanium. Also, condensed ring cyclopentadienyl structures as described previously can be used in providing the metallocene ligands. Specifically, the Cp ring structures may include substituted fluorenyl and indenyl groups.

Preferably, the syndiospecific metallocene catalysts of the present invention exhibit bilateral symmetry of the metallocene ligands when viewed as planar projections of the cyclopentadienyl groups. By the term "bilateral symmetry" as used here, is meant the symmetry of the ligand as viewed through the axes of the substituted Cp groups. For example, ligands having tetra substituted cyclopentadienyl ring pairs would exhibit such bilateral symmetry if the substituents of each ring were the same or if the substituent at the three and four positions of a ring were the same and the substituent at the 2 and 5 positions were the same. However, a ligand with a 2, 3, 4 trisubstituted Cp group or with an indenyl group would not exhibit bilateral symmetry.

Any suitable technique can be used to produce bulky substituted cyclopentadienyl groups which may be lithiated, for example, following a protocol such as disclosed in U.S. Pat. No. 4,892,851, for reaction with a transition metal chloride to form the neutral ligand which is then converted to the cationic state. However, in formulating the neutral metallocene precursors for later conversion to cationic metallocenes of the present invention, the lithiated substituted cyclopentadienyl groups are reacted stepwise with the transition metal salt, e.g., zirconium or titanium tetrachloride, with the product of this reaction reacted with the other dissimilar bulky substituted cyclopentadienyl group. By way of example using the conventions Cp' and C" to designated dissimilar cyclopentadienyl groups having bulky substituents as described above, a lithiated Cp' group may be reacted with zirconium tetrachloride to produce the dicyclopentadienyl (Cp'2) zirconium dichloride. The resulting product may be chlorinated to produce the monocyclopentadienyl zirconium trichloride and this product (Cp'XrC13) then reacted with the lithiated Cp" group to produce the product (Cp') (Cp") ZrC12. Those skilled in the art will recognize that this stepwise reaction formate can be followed to produce metallocene based upon titanium, hafnium, vanadium or other suitable transition metals.

Bulky substituted cyclopentadienyl groups from which the metallocene ligand is formed can be derived by any suitable technique. Starting materials include benzyl alcohol, ketones of substituted cyclopentadienes, e.g., tetraphenylcyclopentadiene-1-ketone and substituted fulvenes. By way of example, pentabenzylcyclopentadiene can be produced by reaction of 5 moles of benzyl alcohol with 1 mole of cyclopentadiene in the presence of particulate sodium. The sodium acts to promote ring aromatization as is well known in the art. Pentaphenylcyclopentadiene can be produced by reaction of tetraphenylcyclopentadienyl-1-ketone with phenyl lithium. This same reaction route can be used to produce other penta-substituted cyclopentadienes. For example, substituted cyclopentadienyl-1-ketone can be reacted with an alkyl lithium such as methyl lithium, ethyl lithium, n-propyl or isopropyl lithium, or normal butyl or tertiary butyl lithium in tetrahydraforan to form the corresponding penta-substituted cyclopentadiene, for example methyl tetraphenylcyclopentadiene and the corresponding ethyl, propyl, isopropyl, butyl, and tertiarybutyl tetraphenylcyclopentadienes. Benzyl tetraphenylcyclopentadiene and pentaphenylcyclopentadiene can also be prepared by this reaction route using phenyl lithium and beryl lithium, respectively.

The reaction of methyl lithium or another alkyl lithium with a tetra substituted dimethylfulvene may be employed to arrive at the bulky substituted cyclopentadienyl group. For example, tetraphenyl-6-dimethylfulvene may be reacted with methyl lithium or ethyl lithium to produce tertbutyl tetraphenyl cyclopentadiene or tertamyl tetraphenyl cyclopentadiene, respectively. The resulting substituted cyclopentadienes can be reacted through the previously described stepwise procedure with a transition metal halide, e.g., titanium hafnium or zirconium tetrachloride to produce the corresponding dichloride in which dissimilar substituted cyclopentadienyl groups are coordinated with the titanium, zirconium or other transition metal in the neutral metallocene complex. It will recognized from the foregoing that numerous metallocene ligands having dissimilar bulky substituted cyclopentadienyl groups can be prepared following the reaction formats indicated above. In addition to those described previously, such ligands include (pentabenzylcyclopentadienyl) (ethyl tetraphenylcyclopentadienyl), (pentabenzylcyclopentadienyl), (propyltetraphenylcyclopentadienyl), (benzyltetraphenyl cyclopentadienyl) (tetrabenzyl cyclopentadienyl) and (isoamyltetraphenylcyclopentadienyl) (methyltetraphenylcyclopentadienyl).

The neutral metallocenes can be converted to the cationic state by any suitable technique. Preferably, such conversion is effected using a trityl compound such as triphenylcarbenium tetrakis (pentafluorolphenyl borate) as described above. Other suitable techniques are disclosed in the aforementioned European Applications 277,003 and 277,004.

As noted previously, the nonbridged cationic metallocene catalysts of formula (2) are stabilized by employing them under low temperature polymerization conditions. For example, polymerization of propylene may be carried out at temperatures of down to about -78° C. or lower using the sterically hindered nonbridged metallocenes of the present invention. The temperatures at which the metallocene catalysts of the present invention are stereorigid can be determined for an individual catalyst based upon nuclear magnetic residence studies as described below. As a practical matter, the substituted cyclopentadienyl groups are at sufficiently low kinetic energy states induced by the substituent groups so as to prevent rotation of the rings about their coordination axes at temperatures which are less than -20° C. Lower temperatures, e.g., less than -40° C., may be required, and much lower temperatures can be used. As indicated previously, the polymerization of propylene can be carried out at temperatures of -78° C. or lower.

The temperatures at which a bulky substituted metallocene catalyst of the present invention becomes stereorigid can be readily determined through nuclear magnetic resonance studies as the temperature of the catalyst is lowered from room temperature condition. The NMR spectrum of a given metallocene having sterically different bulky substituted cyclopentadienyl groups will initially show two sharp identifiable peaks corresponding to the two sharp identifiable peaks corresponding to the two substituted cyclopentadienyl rings. As the temperature is lowered, the peaks will change in characteristics, typically broadening, and measurements can be taken at progressively decreasing temperature values until it is observed that no changes in the NMR spectrum occurs when going from one temperature to the next. At this temperature, the metallocene ligand is in a stereorigid condition in which the substituted cyclopentadienyl groups are no longer rotating.

Having described specific embodiments of the present invention, it will be understood that modification thereof may be suggested to those skilled in art, and it is intended to cover all such modifications as fall within the scope of the appended claims. 

I claim:
 1. A process for syndiotactic propagation of a polymer chain derived from an ethylenically unsaturated monomer comprising:(a) providing a catalyst comprising an unbalanced metallocene cation and a stable noncoordinating counter anion for said metallocene cation, said metallocene cation being characterized by a cationic metallocene ligand having sterically dissimilar ring structures joined to a positively charged coordinating transition metal atom, one of said ring structures being a substituted cyclopentadienyl group and the other of said ring structures being a substituted cyclopentadienyl group which is sterically different from said first cyclopentadienyl group, and both of said cyclopentadienyl groups being substituted with substituent groups which are sufficiently bulky to induce low kinetic energy states to said ring structures so as to prevent rotation of said ring structures about their coordination axes at the temperature of the reaction zone of step (b); and (b) contacting said catalyst in a polymerization reaction zone with an ethylenically unsaturated monomer which contains 3 or more carbon atoms or which is a substituted vinyl compound and maintaining said reaction zone under polymerization conditions to produce syndiotactic polymerization of said monomer.
 2. The process of claim 1 wherein the transition metal of said catalyst is hafnium, zirconium or titanium.
 3. The process of claim 1, wherein said ethylenically unsaturated monomer is a vinyl aromatic compound.
 4. The process of claim 3, wherein said vinyl aromatic compound is styrene.
 5. The method of claim 1, wherein said ethylenically unsaturated monomer is a substituted vinyl compound.
 6. The method of claim 5, wherein said substituted vinyl compound is vinyl chloride.
 7. The method of claim 1, wherein said ethylenically unsaturated monomer is a C₃ -C₄ alpha olefin.
 8. The method of claim 7, wherein said C₃ -C₄ alpha olefin is propylene.
 9. A process for the production of syndiotactic propylene comprising:(a) providing a stereorigid cationic metallocene catalyst characterized by the formula:

    [(CpTx)(CpT'y)MeQk]+.sup.-

wherein:Cp is a cyclopentadienyl ring; each T is the same or different and is a 20 hydrocarbyl radical having from 1-20 carbon atoms; each T' is the same or different and is a hydrocarbyl radical having from 1-20 carbon atoms and selected such that CpTx and CpT'y are sterically different groups and are both at sufficiently low kinetic energy states induced by the substituent group T and T' so as to prevent rotation of said rings about their coordination axes at the temperature of the reaction zone in step (b); Me is a Group 4, 5, or 6 metal from the Periodic Table of Elements; each Q is a hydrocarbyl group having from 1-20 carbon atoms or is a halogen x is from 1 to 5; y is from 1 to 5; k is from 0 to 2; and Pa⁻ is a stable noncoordinating cation. (b) contacting said catalyst with propylene in a polymerization reaction zone and maintaining said reaction zone under polymerization reaction conditions to produce syndiotactic polypropylene.
 10. The process of claim 9 wherein Me is titanium, zirconium or hafnium and K is
 1. 11. The process of claim 10 wherein Q is a halogen or a methyl or ethyl group.
 12. The catalyst of claim 11 wherein Q is a methyl group. 